Read, page 141, link http://www.iz.sp.gov.br/pos/anais.php and http://www.iz.sp.gov.br/pdfs/1326218147.pdf, page 141 - Estimate biological fixation of nitrogen by Arachis pintoi cv. Belmonte

Hi Tersur,

Above answer by Hani is correct.  To derive a prediction equation for estimating biological N fixation (BNF) in the two forage species in your climatic and soil conditions, you need to do some prior research. An example for such work can be found in this short article: ‘BIOLOGICAL NITROGEN FIXATION BY FOUR UNDERSTORY LEGUME SPECIES IN COCONUT PLANTATIONS OF SRI LANKA’ which is posted under my RG profile.  As you can see from the article, we first performed a field experiment using 15N labelled fertilizer, and obtained the data related to above ground and below ground plant dry matter, N concentration in different plant parts and N derived from BNF.  Using this data we performed a regression analysis to derive the equation given in the text.  This equation can be only applied to estimate BNF by these legumes in that particular soil and climatic conditions.

Since the use of 15N is not an easy task and you have requested an easy method, there is a simple method called ‘N difference method’ which is relatively satisfactory for N limiting tropical soils with low soil organic matter content.  With this method, BNF is estimated from the difference in N uptake between a legume crop and non-nitrogen fixing reference crop growing in the same soil.  There are two major assumptions in this method: (1) N contained in the non-nitrogen fixing reference plant is derived only from the soil, and (2) the legume crop and the reference crop assimilate the same amount of soil N.  This second assumption is questionable, but the method can be used where facilities for only total N analysis are available.  It can provide relatively satisfactory results under conditions where soil organic matter content is low.  I have used the ‘N difference method’ (prior to having access to 15N use) when I was in Sri Lanka where soils were low in organic matter and total N content and found it was relatively satisfactory.  Good luck with your research.

-SJ

Valdinei, Hani and Susantha, my appreciation to all of you. I'm grateful.